Drill bits, including roller cone rock bits and percussion rock bits, are employed for drilling rock, for instance as in drilling wells, or for drilling blastholes for blasting in mines and construction projects. The bits are connected to a drill string at one end and typically have a plurality of cemented tungsten carbide inserts embedded in the other end for drilling rock formations.
Drill bits wear out or fail in such service after drilling many meters of bore hole. The cost of the bits is not considered so much as the cost of the bit, per se, as much as it is considered in the cost of drilling per length of hole drilled. It is considered desirable to drill as much length of bore hole as possible with a given bit before it is used to destruction. It is also important that the gage diameter of the holes being drilled remain reasonably near the desired gage. Thus, wear of the bit that would reduce the hole diameter is undesirable. Further, wear of the inserts in the bit during drilling reduces their protrusion from the surface of the drill bit body. The protrusion has a strong influence on the drilling rate. Thus, as the inserts wear out, the rate of penetration may decrease to the extent that it becomes uneconomical to continue drilling. It is therefore quite desirable to maximize the lifetime of a drill bit in a rock formation, both for reducing bit costs and for maintaining a reasonable rate of penetration of the bit into the rock.
Moreover, when a drill bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string for replacing the bit. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. This time can become a significant portion of the total time for completing a well, particularly as the well depths become great. It is therefore quite desirable to maximize the lifetime of a drill bit in a rock formation because prolonging the time of drilling minimizes the lost time in "round tripping" the drill string for replacing bits. Thus, there is a continual effort to upgrade the performance and lengthen the lifetime of those components of a drill bit that are likely to cause a need for replacement.
When a roller cone rock bit is drilling a bore hole, it is important that the diameter or gage of the bore hole be maintained at the desired value. The outermost row of inserts on each cone of a rock bit is known as the gage row. This row of inserts is subjected to the greatest wear since it travels furthest on the bottom of the hole, and the gage row inserts also tend to rub on the side wall of the hole as the cones rotate on the drill bit body. As the gage row inserts wear, the diameter of the bore hole being drilled may decrease below the original gage of the rock bit. When the bit is worn out and removed, a bottom portion of the hole is usually under gage. When the next bit is run in the hole, it is therefore necessary to ream that bottom portion of the hole to bring it to the full desired gage. This not only takes substantial time, but commences wear on the gage row inserts, which again results in an under gage hole as the second bit wears out.
The rate of penetration of a drill bit into the rock formation being drilled is an important parameter for drilling. Clearly it is desirable to maintain a high rate of drilling since this reduces the time required to drill the bore hole, and such time can be costly because of the fixed costs involved in drilling. The rate of penetration decreases when the inserts in the bit become worn and do not protrude from the surface to the same extent they did when drilling commenced. The worn inserts have an increased radius of curvature and increased contact area on the rock. This reduces the rate of penetration.
Thus, it is important to maximize the wear resistance of the inserts in a drill bit to maintain a high rate of penetration as long as possible. It is particularly important to minimize wear of the gage row inserts to maximize the length of hole drilled to full gage.
A significant improvement in the life expectancy of drill bits, including roller cone and percussion rock bits, involves the use of cemented metal carbide inserts put into the drill bit for crushing rock on the bottom of the bore hole. Naturally, cemented metal carbide, such as cobalt cemented tungsten carbide, offered improved wear resistance over steel along with sufficient toughness to withstand the forces encountered during drilling. Since the advent of cemented metal carbide inserts in rock drilling, much effort has been devoted to improving both the wear resistance and toughness of the inserts. Wear resistance is important to prevent the insert from simply wearing away during drilling. Toughness is important to avoid inserts breaking off due to the high impact loads experienced in drilling.
A more recent development in drill bit inserts has been the use of a layer of polycrystalline diamond (PCD). In particular, "enhanced" inserts, as they are called, have been fabricated which include an insert body made of cobalt bonded tungsten carbide and a layer of polycrystalline diamond directly bonded to the protruding head portion of the insert body. The term polycrystalline diamond generally refers to the material produced by subjecting individual diamond crystals to sufficiently high pressure and high temperature that intercrystalline bonding occurs between adjacent diamond crystals. Naturally, PCD offers the advantage of greater wear resistance. However, because PCD is relatively brittle, some problems have been encountered due to chipping or cracking in the PCD layer.
U.S. Pat. No. 4,694,918 discloses roller cone rock bits and inserts therefor, which inserts include a cemented metal carbide insert body, an outer layer of polycrystalline diamond, and at least one transition layer of a composite material. The composite material includes polycrystalline diamond and particles of precemented metal carbide. Although this transition layer between the outer layer of PCD and the head portion has been found to extend the life expectancy of PCD rock bit inserts by reducing the incidence of cracking and chipping, the current enhanced inserts still are not optimum for drilling rock formation with high compressive strength. Although the PCD layer is extremely hard and therefore resistant to wear, the typical mode of failure is cracking of the PCD layer due to high contact stress, lack of toughness, and insufficient fatigue strength. A crack in the PCD layer during drilling will cause the PCD layer to spall, or delaminate, exposing the head portion of the insert to significantly increased wear. A crack in the PCD layer may propogate through the cemented tungsten carbide body of the insert and cause complete failure of the insert. It is therefore desirable to provide inserts that are not only hard, to resist wear, but also tough enough and strong enough to drill through rock formation with high compressive strength without breakage or delamination of the PCD layer.